The reduction of emission components from the exhausts of internal combustion engines is one of the objectives sought by virtually every manufacturer of such engines. For some time it has been recognized that one of the best methods for controlling emissions is to supply fuel and air to the engine cylinders in a ratio which allows complete combustion under all operating conditions, thereby severely limiting the production of components which require removal from the engine exhaust. If the air fuel ratio is controlled carefully enough, the need for apparatus to remove emissions to achieve acceptable emission control can be entirely eliminated. One approach to achieving this desirable air-fuel ratio has been to provide a fuel metering system which is responsive to changes in pressure within the system. U.S. Pat. Nos. 2,894,735 to Zupancic, 3,726,263 to Kemp and 4,015,571 to Stumpp all disclose earlier attempts to regulate the air fuel mixture in internal combustion engines through such a system. In U.S. Pat. No. 2,894,735 Zupancic describes a fuel metering system responsive to manifold pressure and in U.S. Pat. No. 4,015,571 Stumpp discloses a fuel metering system including a throttle which is responsive to pressure changes in the entire fuel system once the desired air-fuel ratio is chosen. Kemp, in U.S. Pat. No. 3,726,263, describes a fuel flow control which provides a diaphragm subjected to manifold pressure on one side to modulate fuel flow to the engine in response to changing manifold pressure while the reverse side of the diaphragm is connected with a fuel drain line so that fuel leaking within the fuel flow control is returned to the fuel tank.
Systems of the type described above can be used in engine fuel systems of the type having a common rail supplying the cylinder injectors with a varying fuel pressure to control engine speed. However, it has been found that the fuel to air ratio supplied to the engine cylinders in such systems is not always maintained at the ideal level even when an air fuel control is employed to modulate fuel flow to the engine in response to changing manifold pressure. For example, some limitation appears to be required in the rate of increase in fuel flow to the engine in response to increasing manifold pressure. Without such a limitation, a highly undesirable fuel to air ratio may be supplied to the engine cylinders under certain operating conditions. The limitation provides a beneficial reduction in combustion noise as well as smoke. On the other hand, a very quick response to decreasing intake manifold pressure is desirable in order to reduce immediately the fuel flow to the engine as soon as the manifold pressure begins to decrease. To achieve this desirable transient response, it has been known to provide an air attenuator valve assembly in the air signal line extending between the intake manifold and the air fuel control valve. The attenuator valve assembly (consisting of a check valve and restriction orifice connected in parallel) allows free flow of air through the check valve and the air signal line upon decreasing manifold pressure but requires return flow of air in the air signal line to pass through the restriction orifice to limit thereby the transient response of the air fuel control. Although such attenuator assemblies provide the desired fuel flow modulating characteristics in response to changes in manifold pressure so long as they remain operable, the trouble free operating life of this type of assembly is normally insufficient from a commercial standpoint. In particular, such assemblies are susceptible to clogging by air borne particles. Filtering of the air has not been shown to present a satisfactory solution. None of the prior art systems which employ flexible diaphragm means to separate high and low pressure areas has fully solved the problems presented by leaks in the diaphragm and the resulting presence of fuel in the manifold and subsequent effects which could accompany such a leak. Kemp, in U.S. Pat. No. 3,726,263, does suggest a technique for recycling leakage fuel by connecting one side of a diaphragm operator to a fuel drain line but does not suggest a technique for simultaneously modulating the transient response of the fuel control valve.